Springless time-lag fuses for motor circuits



F. J. KOZACKA 3,143,615

SPRINGLESS TIME-LAG FUSES FOR MOTOR CIRCUITS Aug. 4, 1964 Filed April 6,1962 Patented Aug. 4, 1964 3,143,615 SPRINGLESS TIME-LAG FUSES FOR MOTGRCIRCUITS Frederick J. Kozacka, South Hampton, N.H., assignor to TheChase-Shawmut Company, Newburyport, Mass.

Filed Apr. 6, 1962, Ser. No. 185,618 8 Claims. (Cl. 200-120) Thisinvention has reference to electric fuses, and more particularly totime-lag fuses.

It is one object of this invention to provide springless time-lag fusesadapted to be inserted into motor circuits.

This application is a continuation-in-part of my copending patentapplication Ser. No. 21,044, filed June 13, 1961, for Time-Lag Fuses,now U.S. Patent 3,029,328, issued April 10, 1962.

It is another object of this invention to provide timelag fuses whosegeometry issimilar to, and whose mode of operation is substantially thesame as that of, the fuses disclosed in my above U.S. Patent 3,029,328,but which have improved operating characteristics, particularly inregard to the arc voltage generated incident to blowing of the fuses asa result of major fault currents.

Another object of the present invention is to provide time-lag fuseswhose fuse links have a .center portion of increased length and endportions of reduced length and which fuses have substantially the sametime-current curves as the time-lag fuses described in U.S. Patent 3,-029,328, but are capable of generating substantially higher arc voltagethan the fuses described in the above patent.

Another object of this invention is to provide electric time-lag fusessimilar to those shown in U.S. Patent 3,- 029,328, but having moreeffective arc voltage generating means and more compact heat dam meansor heating means immediately adjacent the terminals thereof. Anotherobject of the invention is to provide springless time-lag fuses of theterminal cap type adapted for relatively low current ratings, e.g.current ratings of 30 to. 60 amps. and capable of delaying blowing atthe oc and axially outer portions exceeding in length the length of saidaxially inner portion. The axially outer portions are bent around theedges or rims of casing A and turned upon the outer surface thereof. Thestructure of FIGS. 1 and 1a further comprises an overlay C on theaxially inner portion of fuse link B of a metal having a lower fusingpoint than the base metal of link B adapted to sever the axially innerportion of link B by a metallurgical reaction involving diffusion ofmetals when said lower fusing point is reached. FIGS. 1 and 1a do notshow a pair of terminal caps which are present in the completed fusestructure, i.e. which are mounted upon the ends of casing A upon theaxially outer portions of link B which are situated on the outer surfaceof casing A. For additional details of the structure of FIGS. 1 and 1areference may be had to the aforementioned U.S. Patent 3,029,328. Itwill also be understood that the casing of the structure of FIGS. 1 and1a will normally be filled with pulverulent arc-quenching'filler as, forinstance, gypsum or chalk, not in FIGS. 1 and 1a.

'Fuses for the protection of motor circuits require relatively longtime-lags at currents in the order of five times the current rating ofthe fuse. To be more specific, at currents in the order of five timesthe current rating of the fuse the time-lag should be in the order of 10sec. Such time-lags could not be achieved heretofore in fuses havingsmall current ratings by springless fuse structures unless resorting tothe structure of FIGS. 1 and 1a. In that structure the length of thetime-lag is predicated upon the coaction of a number of factor such asthe substantially tubular geometry of the axially inner portion of thefuse link, the presence of overlay C having a relativel large heatabsorbing capacity, the nature of the arc-quenching filler, and to acritical extent the length of the ends of the fuse link of which each isin the form of four outcurrence of transient inrush currents such asfound in motor circuits upon starting of a motor.

Other objects and advantages of the invention will, in part, be obviousand in part appear hereinafter.

For a more complete understanding of the invention reference may be hadto the following detailed description thereof taken in connection withthe accompanying drawings wherein FIG. 1 is a diagrammatic illustrationof the basic structure of U.S. Patent 3,029,328 shown in an axialsection;

FIG. 1a is a cross-section of the structure of FIG. 1;

FIG. 2 is a diagrammatic illustration of the basic structure of a fuseembodying the present invention shown in an axial section;

FIG. 2a is a cross-section of the structure of FIG. 2;

wardly flaring tabs defining a space in the shape of a truncated cone.These ends function as thermal insulators for the relatively shortaxially inner portion of the fuse link, tend to reduce thecurrent-carrying capacity of the latter and thus to achieve a relativelysubstantial time-lag for fuses having a current rating which may be, forinstance, in the order of 30 to amps. The structure of FIGS. 1 and lahas, however, one significant limitation which consists in that asubstantial portion of the total length of the fuse structure isoccupied by the long radially outwardly flaring thermal insulator endsof the fuse link at the expense of the arc-voltage generating centerportion of the fuse link. As a result, the arc-voltages generated by thefuse link by its coaction with a pulverulent arcquenching filler arerelatively low. This tends to limit FIG. 3 is a modification of thestructure shown in FIG.

Referring now to the drawings, and more particularly to FIGS. 1 and lathereof, reference character A has been applied to indicate a tubularcasing of insulating material, e.g. vulcanized fiber. Fuse link B madeof a sheet metal having a relatively high fusing point is arrangedwithin casing A. Fuse link B comprises an axially inner portion bent toenvelop the longitudinal axis of casing A the capacity of the fusestructure to interrupt very high fault currents, particularly if thecircuit voltage is relatively high, say 600 volts.

It is one object of this invention to so modify the structure of FIGS. 1and In that the time-lag characteristics thereof remain unchanged, butthat its ability to generate relatively high are voltages issubstantially increased. This is achieved according to this invention byshorten-j ing the outwardly flaring ends of the fuse link and byincreasing the relative length of the axially inner portion of the fuselink and by providing the axially inner portion of the fuse link with anincreased number of points of reduced cross-sectional area which formseries breaks on blowing of the fuse at relatively high currents, e.g.

currents in the order of 20 times, or 40 times, the rated current of thefuse.

It has been found to be possible to substantially reduce the length ofthe axially outer ends of the fuse link without substantially changingthe time-current curveof the fuse structure by reducing thecross-sectional area of the ends of the fuse link. FIGS. 2 and 2a show astructure comprising a tubular casing A of insulating material, and afuse link B having an axially inner portion bent to envelop thelongitudinal axis of easing A. The axially inner portionof fuse link Bis in the shape of a prism which is triangular in cross-section. Theends of the link are formed by tabs whose total cross-section, orcross-sectional area, is substantially less than the crosssection, orcross-sectional area, of the axially inner portion of fuse link B. Theterm cross-section of the axially inner portion of fuse link B refers tothe normal crosssection of the axially inner portion rather than to thecrosssection at the points of reduced cross-section formed by transverselines of circular perforations. The aggregate cross-section of the threetabs at each end of the axially inner portion of fuse link B is slightlylarger than'the cross-section of the axially inner portion of fuse linkB at the points where the latter has a reduced cross-section by virtueof the presence of a transverse line of circular perforations. Theaggregate cross-section of the tabs forming the ends of fuse link Bcannot be further reduced because in case of high fault currents suchreduction would result in a transfer of the points of are initiationfrom the points of reduced cross-section of the axially inner portion oflink B to the end tabs of fuse link B. This, in turn, would be conduciveto failure of the fuse structure.

FIG. 2 indicates that the cross-section of each end tab is smaller thanthe cross-section of each side of the prism immediately adjacent to therespective end tab. The structure of FIGS. 2 and 2a is provided with anoverlay C of a metal having a relatively low fusing point operating as aheat absorber and as a means for severing the axially inner portion ofthe fuse link C after predetermined timedelays.

In the structure of FIGS. 3 and 3a the axially inner portion of sheetmetal fuse link A" is in the shape of a prism having four sides andbeing square in crosssection. Each end of the axially inner portion offuse link B" has but two end tabs. Each of these end tabs has about thesame cross-section as one of the four sides of the axially inner portionor, in other words, the cross-section of the axially outer end tabs isabout one half of the cross-section of the axially inner portion of linkB" which is bent to envelop the longitudinal axis of easing A" andsupports overlay C" of a low fusing point metal.

It will be noted that the link structure of FIGS. 2 and 2a is providedwith three transverse lines of circular perforations, and that the linkstructure of FIGS. 3 and 3a is provided with six such lines ofperforations, thus greatly increasing the points of series breaksformed, and the arc voltage generated, incident to the blowing of thefuse at major fault currents. While the reduction of the number of endtabs, and the reduction of their aggregate crosssection, respectively,is primarily intended to allow an increase of the arc voltage incidentto blowing on severe fault currents, a secondary desirable result ofthese features consists in that they facilitate the filling of thecasing of the fuse with a pulverulent arc-quenching filler during theprocess of manufacturing of the fuse structure.

Referring now to FIGS. 4-6, reference character A" has been applied toindicate a tubular cylindrical casing of insulating material, e.g.vulcanized fiber, closed on both ends thereof by terminal caps 1. CasingA" is filled with a pulverulent arc-quenching filler 2 having a smallerheat conductivity than quartz sand. The use of such a filler isindicated wherever it is required to achieve very high time-delays inthe current range of about five times the rated current of the fuse.Where the time-delay requirements are less stringent quartz may be usedas an arcquenching filler yet, in this case, the casing of the fuseought to be made of an insulating material which is more heat resistantthan vulcanized fiber. A pair of terminal 4 caps of pressed sheet metalis mounted on the ends of easing A". The axially inner ends of caps 1are preferably crimped radially inwardly to firmly secure caps 1 tocasing A". Casing A houses a fuse link B" of a sheet metal having arelatively high fusing point. Link B" may be of copper, or silver. It issubmersed in the pulverulent arcquenching filler 2 and includes theaxially inner or center portion 6 situated in the center region ofeasing A and bent to form four radially juxtaposed surface elementsextending parallel to the generatrices of easing A". These four surfaceelements are in the shape of a prism having four sides and being squarein cross-section. Fuse link B" is also fashioned to form outwardlyflaring ends 7 bent around the rims 4 of casing A and turned upon theouter surface thereof and covered by caps 1. The ends 7 on each end ofthe center portion 6 have a smaller aggregate cross-section than thecenter portion at the points thereof where its cross-section is notreduced by perforations. The portions of the ends 7 situated between thecenter portion 6 and the axially outer rims 4 have an aggregate lengthless than the center portion 6. In FIG. 4 the length of the centerportion has been indicated by the reference character S and theaggregate length of the ends '7 situated between center portion 6 andrims 4 has been indicated by 2s. It will be apparent that S 2s. Thelength of each cap is s and it is apparent from FIG. 4 that se s, i.e.that the length of each end 7 situated between center portion 6 and anaxially outer rim 4 is approximately equal to the length s of one of thecaps. While this is the ultimate in the process of shortening the endsand increasing the relative length of the arc voltage generating centerportion 6, it is generally desirable to impart to s a value slightlyexceeding the length s of caps 1.

It Will be apparent from FIG. 4 that a washer of asbestos or of a likefibrous heat resistant material is interposed between rims 4 and theinside of caps 1.

Link B" is provided with an overlay C of a metal having a relatively lowfusing pointe.g. tin-coextensive with the permimeter of center portion 6for severing center portion 6 by a metallurgical reaction involvingdiffusion of metals upon reaching of said relatively low fusing point bysaid overlay.

It will be noted from FIGS. 4-6, inclusive, that the number of end tabsis less than the number of the sides of the prismatic center portion 6,the former being two at each end of center portion 6, and the number ofthe sides of center portion 6 being four.

As clearly shown in FIGS. 4 and 5 the center portion 6 of fuse link B"is provided with six transverse lines 3 formed by circular perforations.The constituent perforations of each line 3 are arranged along a familyof parallel lines extending in a direction longitudinally of casing A"and center portion 6 of fuse link B" is bent along lines of said familyto form a substantially prismatic structure. In other words, each of thefour edges of the prismatic center portion 6 of fuse link B" is formedby one member of the aforementioned family of lines. It is relativelyeasy to bend the center portion 6 of fuse link B" along said family oflines since'each of them is a locus of minimal section in a directionlongitudinally of portion 6 of link B".

In the process of manufacturing a fuse structure of the kind shown inFIG. 3 the center portion 6 of link B must be positioned exactly in thecenter region of easing A" and the end tabs '7 must be bent around rims4 of casing A" exactly at the right points. This exacting task isrendered quite easy even for relatively unskilled workers by theprovision of an index mark 8 on each end tab 7 indicating the pointwhere the mark 8 must register with rim 4 in order that center portion 6be properly centered, and indicating the several points where endtabs 7must be bent around rims 4.

On occurrence of transient overloads of relatively short durationoverlayC" will not meltrand, therefore, center portion 6 of link B" willremain intact.

If an overload current is of excessive duration overlay C will melt,cause formation of a pair of series breaks along the twov transverselines 3 of perforations between which overlay C" is arranged, and thisresults in generation of a sufficiently high arc-voltage to cause rapidinterruption of the overloaded circuit.

Short-circuit currents result in the formation of six series breaks,each at one of the six transverse lines 3 of circular perforations.

It will be understood that although but two embodiments of the inventionhave been illustrated and described in detail, the invention is notlimited thereto. It Will also be understood that the structuresillustrated may be modified without departing from the spirit and scopeof the invention as set forth in the accompanying claims.

I claim as my invention:

1. A time-lag fuse comprising in combination:

(a) a tubular cylindrical casing of insulating material;

(b) a pulverulent arc-quenching filler inside said casa pair of terminalcaps each mounted upon one of the ends of said casing and closing saidcasing;

(d) a fuse link of sheet metal having a relatively high fusing pointarranged inside said casing and submersed in said filler, said fuse linkincluding a center portion situated in the center region of said casingand bent to form radially juxtaposed surface elements extending parallelto the generatrices of said casing, and said fuse link being fashionedto form outwardly flaring ends bent around the axially outer rims ofsaid casing and turned upon the outer surface thereof and covered bysaid pair of caps, said ends on each end of said fuse link being smallerin aggregate crosssection than said center portion and the portion ofsaid ends situated between said center portion and said axially outerrims having an aggregate length less than said center portion; and

(e) an overlay of a metal having a relatively low fusing pointcoextensive with the perimeter of said center portion and supported bysaid center portion for severing said center portion by a metallurgicalreaction upon reaching of said relatively low fusing point by saidoverlay.

2. A time-lag fuse as specified in claim 1 wherein said center portionis substantially in the shape of a prism which is square incross-section and wherein there are less than four of said outwardlyflaring ends on each end of said center portion.

3. A time-lag fuse as specified in claim 1 wherein there are but twooutwardly flaring ends on each end of said center portion, and whereinthe portion of said ends situated between said center portion and saidaxially outer rims of said casing have a length but slightly exceedingthe length of each of said pair of terminal caps.

4. A time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pulverulent arc-quenching filler inside said casing;

(c) a pair of terminal caps each mounted upon one of the ends of saidcasing and closing said casing; (d) a fuse link of a sheet metal havinga relatively high fusing point arranged inside said casing and submersedin said filler, said fuse link being fashioned to form a substantiallytubular structure situated in the center region of said casing incoaxial relation thereto and to further form outwardly flaring ends bentaround the axially outer rims of said casing and turned upon the outersurface thereof and covered by said pair of caps, said ends beingsmaller in crosssection than said tubular structure and the portion ofsaid ends situated between said tubular structure and said rims having ashorter aggregate length than said tubular structure; and 1 (e) asubstantially annular overlay of a metal having a relatively low fusingpoint supported by said tubular structure for severing said fuse link bya metallurgical reaction upon reaching of said relatively low fusingpoint by said overlay. V

5. A time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(12) a pulverulent arc-quenching filler having a smaller thermalconductivity than quartz-sand inside said casing;

(c) a pair of terminal caps each mounted upon one of the ends of saidcasing and closing said casing;

(d) a fuse link of sheet metal having a relatively high fusing pointarranged inside said casing and submersed in said filler, said fuse linkincluding a center portion situated in the center region of said casingand provided with a plurality of transverse lines of perforations, theconstituent perforations of each said transverse lines being arrangedalong a family of parallel lines extending in a direction longitudinallyof said casing and said center portion being bent along lines of saidfamily of lines to form a substantially prismatic structure coaxial tosaid casing, and said fuse link being fashioned to form outwardlyflaring ends bent around the axially outer rims of said casing andturned upon the outer surface thereof and covered by said pair of caps,said ends on each end of said center portion being smaller in aggregatecross-section than said center portion and the portion of said endssituated between said center portion and said axially outer rims havingan aggregate length less than said center portion thereof; and

(e) an overlay of a metal having a relatively low fusing pointcoextensive with one of said transverse lines of perforations of saidcenter portion for severing said center portion by a metallurgicalreaction upon reaching of said relatively low fusing point by saidoverlay.

6. A time-lag fuse as specified in claim 5 wherein each of saidtransverse lines of perforations defines a region of reducedcross-section having a predetermined cross-sectional area, and whereinthe cross-section of said outwardly flaring ends on each end of saidcenter portion is but slightly larger than said predeterminedcross-sectional area.

7. A time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a fuse link of a sheet metal having a relatively high fusing pointarranged inside said casing and including a substantially tubular centerportion positioned in the center region of said casing in coaxialrelation thereto, said center portion being substantially square incross-section and provided with a plurality of trans verse lines ofperforations;

(c) a substantially annular overlay of a metal having a relatively lowfusing point supported by said center portion adjacent one of saidplurality of lines of perforations for severing said center portion by ametallurgical reaction upon reaching of said relatively low fusing pointby said overlay; and

(a') means integral with said center portion conductively connectingeach end of said center portion and each of said pair of terminal capsat but two points, said connecting means including a pair of tabsangularly displaced degrees on each end of said center portion andforming extensions of two opposite sides of said center portion, saidpair of tabs flaring radially out from said center portion, being bentaround the axially outer rims of said casing upon the outer surfacethereof and covered by said pair of caps, and two opposite sides of eachend of said center portion being tab-less.

8. A time-lag fuse as specified in claim 7 wherein each of said pair oftabs forms a shoulder at the point thereof bent around one of saidaxially outer rims of said casing.

(References on following page) References Cited in the file of thispatent v2,041,590 UNITED STATES PATENTS 1,215,999 Schweitzer Feb. 13,1917 1,443,886 Sands 2 Jan. 30, 1923 5 1,478,109 Ellison Dec. 18, 1923 1338,890 1,774,252 507,334

Bussmann Aug. 26, 1930 8 Brown May 19, 1936 Alberga et a1 July 11, 1939Kozacka Apr. 10, 1962 FOREIGN PATENTS Switzerland 'I11ly 31, 1959 GreatBritain June 13, 1939

1. A TIME-LAG FUSE COMPRISING IN COMBINATION: (A) A TUBULAR CYLINDRICALCASING OF INSULATING MATERIAL; (B) A PULVERULENT ARE-QUENCHING FILLERINSIDE SAID CASING; (C) A PAIR OF TERMINAL CAPS EACH MOUNTED UPON ONE OFTHE ENDS OF SAID CASING AND CLOSING SAID CASING; (D) A FUSE LINK OFSHEET METAL HAVING A RELATIVELY HIGH FUSING POINT ARRANGED INSIDE SAIDCASING AND SUBMERSED IN SAID FILLER, SAID FUSE LINK INCLUDING A CENTERPORTION SITUATED IN THE CENTER REGION OF SAID CASING AND BENT TO FORMRADIALLY JUXTAPOSED SURFACE ELEMENTS EXTENDING PARALLEL TO THEGENERATRICES OF SAID CASING, AND SAID FUSE LINK BEING FASHIONED TO FORMOUTWARDLY FLARING ENDS BENT AROUND THE AXIALLY OUTER RIMS OF SAID CASINGAND TURNED UPON THE OUTER SURFACE THEREOF AND COVERED BY SAID PAIR OFCAPS, SAID ENDS ON EACH END OF SAID FUSE LINK BEING SMALLER IN AGGREGATECROSSSECTION THAN SAID CENTER PORTION AND THE PORTION OF SAID ENDSSITUATED BETWEEN SAID CENTER PORTION AND SAID AXIALLY OUTER RIMS HAVINGAN AGGREGATE LENGTH LESS THAN SAID CENTER PORTION; AND (E) AN OVERLAY OFA METAL HAVING A RELATIVELY LOW FUSING POINT COEXTENSIVE WITH THEPERIMETER OF SAID CENTER PORTION AND SUPPORTED BY SAID CENTER PORTIONFOR SEVERING SAID CENTER PORTION BY A METALLURGICAL REACTION UPONREACHING OF SAID RELATIVELY LOW FUSING POINT BY SAID OVERLAY.